The present invention relates generally to the field of electroless metal plating. In particular, the present invention relates to the field of electroless gold plating.
Immersion or displacement plating is an electroless plating process, but is given a separate classification in the art. In immersion plating, deposition is by displacement of an elemental metal from a substrate by metal ions in a plating solution. In electroless plating deposition takes place primarily by autocatalytic reduction of metal ions from solution. Such electroless plating requires the presence of a reducing agent.
Immersion plating does not employ an external electric current but rather is an electrochemical displacement reaction which is driven by the position of the substrate metal in the electromotive series relative to the metal to be deposited from solution. Plating occurs when the dissolved metal ions in a plating bath are displaced by a more active (less noble) metal that is contacted with the plating bath.
In the manufacture of printed wiring boards, solderable finishes are typically applied to printed wiring board substrates having pads and/or through holes exposed through a mask, such as a soldermask. Such solderable finishes are often applied by immersion plating as electroless plating can also deposit metal on the surface of the mask, which is undesirable. As an immersion plating reaction is driven by the difference in electrochemical potentials, plating will only occur at areas of exposed metal. For example, U.S. Pat. No. 5,143,544 (Iantosca) discloses a solution for immersion plating a tin-lead alloy suitable as a solderable finish on a printed wiring board substrate. However, there is a growing demand for more environmentally acceptable alternatives to lead for use in printed wiring board manufacture. Thus, the use of lead and lead alloys in electronic components faces an uncertain future. See, for example, U.S. Pat. No. 5,536,908 (Etchells et al.).
Gold is a more environmentally acceptable alterative to lead and has long been used in the electronics industry as a metal for contact surfaces because of its low electrical resistivity and its inertness to attack by corrosive substances. Such gold deposits have typically been plated using electroless or immersion gold plating baths. In particular, gold has long been used over a nickel undercoat to provide a solderable finish. Typically, the nickel undercoat is electrolessly applied while the gold is immersion deposited. Such processes are referred to as electroless-nickel-immersion-gold or “ENIG.”
Electroless gold plating baths contain a reducing agent. Typical reducing agents are thiourea and alkyl thiourea derivatives, enol-containing compounds such as ascorbic acid (see U.S. Pat. No. 4,481,035 to Andrascek et al.), and boron-containing compounds such as alkylboranes and borohydrides. These conventional plating baths have certain drawbacks. For example, baths containing thiourea as the reducing agent must be heated to about 80° to 90° C. in order to achieve acceptable deposition rates. Such temperatures are too high for use with some electronics packaging materials. Also, at such temperatures the plating solutions can become unstable and spontaneously form fine particles of gold throughout the solution instead of producing gold deposits only on the desired substrate. When boron-containing compounds are used as the reducing agent, such compounds first undergo a hydrolysis reaction whose rate increases with temperature. Much of the boron-containing reducing agent is consumed in undesired side reactions making control of its concentration quite difficult.
One conventional form of electroless gold plating bath is thiosulfate ion based, stabilized with sulfite ions. Such baths are typically unstable when operated at a pH of 6 or below as sulfur dioxide is liberated from the bath under these pH conditions. It is known that the thiosulfate ion decomposes in acid solution to give elemental sulfur and sulfite ions. When an aqueous solution of sodium thiosulfate is adjusted to a pH of about 4 to 5, the solution will turn cloudy due to the formation of elemental sulfur. However, if sodium sulfite is also added to the above solution, elemental sulfur will not form and the solution will be stable and clear. Sodium sulfite has, therefore, been used in prior art metal plating solutions and sodium thiosulfate to stabilize the solution. The problem with using sodium sulfite, however, is that the sulfite ion itself is not stable in mildly acidic solutions, such that sulfur dioxide is slowly formed and liberated from the solution. The more acidic the solution, the faster the rate of sulfur dioxide formation will be. This leads to high consumption of sodium sulfite and instability of the metal thiosulfate complex in acidic solutions.
Immersion gold plating baths avoid many of the above reducing agent-derived drawbacks. However, such immersion plating baths typically require high plating temperatures, such as about 70° C. or greater, for proper operation and do not deposit thick gold layers. Such high temperatures are often incompatible with some electronics packaging materials. Hybrid electroless gold baths that do not automatically deposit gold and do not solely deposit gold by displacement plating have been proposed. While such hybrid gold baths may overcome problems of prior art electroless and immersion gold plating baths, the resulting gold deposits are not sufficiently adherent to substrates, such as nickel or nickel-coated substrates.
European Patent EP 1 021 593 B1 (Backus et al.) discloses an electroless gold plating bath that is particularly suitable for depositing gold on palladium In this patent, gold layers are electrolessly deposited on continuous palladium layers as such plating baths fail to plate on other metals, such as nickel.
Thus, there is a need for methods of depositing sold layers having good adhesion to substrates, particularly to nickel or nickel-coated substrates, while overcoming the problems of conventional electroless gold plating baths.